Amid the rising popularity for digital image devices such as digital cameras is a demand for increasingly higher picture resolution and for increasingly compact designs of such devices. Due to the interior space constraints in the housings of the compact designs, it is necessary to reduce the sizes of the electronic circuits in the device, including the image sensor. However, upon shrinking the size of the image sensor, a tradeoff must be made between resolution and the signal levels outputted from the image sensor. If the resolution is kept the same upon reducing the size of the image sensor, the size of each pixel must be proportionately reduced. Smaller pixels reduce the amount of charge that can be collected by each pixel during image exposure, which in turn reduces the sensitivity of the image sensor. Although the reduced sensitivity effect can be offset by increasing the integration (exposure) time, this is an undesirable “solution” because increasing integration time also increases the potential for obtaining a blurred image if there is any movement by the image subject or the device during exposure. On the other hand, in order to maintain the same sensitivity without having to increase integration time, the pixels must be made larger, which limits the resolution.
One solution towards achieving both a more compact size and high image quality is disclosed in “CMOS Image Sensor Overlaid with a HARP Photoconversion Layer,” by T. Watabe, et al., published in the Program of the 1999 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, pp. 211-214. In this image sensor, which is shown in FIGS. 1A and 1B, the pixel circuit 902 is overlaid with a stacked charge multiplying photoconversion layer, such as a high-gain avalanche rushing amorphous photoconductor (“HARP”) photo-conversion layer 904 for amplifying the light signal produced by each pixel.
When a photon 906 hits the upper surface 908 of the HARP layer 904, a charge 910 in the form of holes is generated and amplified to many times its original level while being propelled through the HARP layer 904 to the bottom side 912. The pixel circuit 902 is electrically connected to the bottom side 912 of the HARP layer 904 such that the amplified light signal 910, upon reaching the bottom side 912 of HARP layer 904, is conducted into the pixel circuit 902 as electrical charge. The charge accumulates at a storage node 914 in the pixel circuit until the pixel data is read out by activating the gate of a row select switch 916. The amount of charge accumulated at the node 914, which is proportional to the intensity of light 906 detected, is read out. In this manner, the image sensor of FIGS. 1A and 1B allows each pixel to capture image data with an intensity and sensitivity equivalent to that attainable by significantly larger pixels which do not have the avalanche multiplication capability. As a result, use of a HARP layer enables the image quality to be improved without having to increase the size of the image sensor array.
In order to obtain avalanche multiplication in the HARP layer, an electric field of about 106 V/cm is required, which is achieved by applying an operating voltage of between 50-100 V to the HARP layer. In a typical HARP image sensor, voltages of less than about 8 V are used in the pixel circuit connected beneath the HARP layer, with the pixel circuit generally having a breakdown voltage of around 20 V. When the intensity of the incident light on the image sensor is at the upper end of the detection range for the charge multiplying photoconversion layer, the voltage level accumulating at the storage diode beneath the HARP layer approaches the level of the operating voltage applied to the HARP layer. Thus, voltages of 50-100 V may be applied to the storage diode when the image sensor is exposed to a strong light, resulting in a breakdown of the readout components of the pixel circuit.
To address this problem, attempts have been made to build a pixel circuit having a higher breakdown tolerance. An example of such a high tolerance pixel circuit is disclosed in the article by T. Watabe et al. mentioned above, in which the pixel circuit is constructed as MOS transistor having a double drain structure. This structure is shown in FIG. 2, in which the n-doped drain formed in the p-doped silicon layer 922 includes a low-dose n− region 924 surrounding a conventional high-dose n+ region 926. The double drain MOS transistor structure was shown to achieve an endurance voltage up to just under 60V. However, a special MOS fabrication process is required for forming the double drain MOS transistor, and the size of the MOS transistor makes it difficult to attain small pixel sizes for high resolution image sensors.